Targeted degradation of proteins has previously been achieved through strategies harnessing the ubiquitin proteasome system (UPS). In particular, Proteolysis Targeting Chimeric molecules (PROTACs) have been described in the art which are heterobifunctional compounds composed of a target protein-binding ligand and an E3 ubiquitin ligase ligand that induce proteasome-mediated degradation of the target protein via their recruitment of E3 ubiquitin ligase and subsequent ubiquitination. Such compounds are capable of inducing the inactivation of a target protein upon addition to cells or administration to an animal or human, and therefore have been proposed for the treatment of disease by removing pathogenic or oncogenic target proteins.
Chimeric antigen receptors (CARs) are artificial T-cell receptors that are at the forefront of modern personalised therapies (Lee et al. (2012) Clin. Cancer Res., 18(10): 2780-90). They are being developed to treat cancers in patients that are resistant to conventionally available therapies and use a patient's own immune cells to combat the disease. The immune cells are genetically engineered ex vivo to express a CAR (CAR-T cells) specific to a tumour antigen, and the cells are subsequently transferred back to the patient. CARs reside on surfaces of T cells and consist of intracellular and extracellular domains which are separated by a transmembrane domain. The extracellular domain harbours a target binding region (e.g. a single chain variable fragment) that is directed towards an antigen solely expressed on diseased cells. The intracellular domain (usually CD3ζ-CD28 or CD3ζ-41BB) faces the cytosol and transmits an activation signal to the T cell after the antigen is bound to the target binding region on the surface of the cell. Active signalling of CAR-T cells leads further to the killing of the diseased cells.
The development of CARs has comprised three generations so far. The first generation CARs comprised target binding domains attached to a signalling domain derived from the cytoplasmic region of the CD3zeta or the Fc receptor gamma chains. First generation CARs were shown to successfully retarget T cell killing to the selected target, however, they failed to provide prolonged expansion and antitumor activity in vivo. The second and third generation CARs have focussed on enhancing modified T cell survival and increasing proliferation by including additional signalling domains from co-stimulatory molecules, such as CD28, OX-40 (CD134) and 4-1BB (CD137).
However, a safety concern of this promising therapy has arisen through potential cross-reactivity to vital organs such as the lung. Indeed, during clinical trials, both on-target as well as off-target off-tumour toxicities have been observed in patients treated with CAR-T cells and fatalities have been reported with CAR studies (Morgan et al. (2010) Mol. Ther., 18(4): 843-51). These toxicities are difficult to predict in animal or non-primate models, and in contrast to small molecules and biologics, CAR-T cells are living-drugs that have unique pharmacokinetic (PK) profiles and pharmacodynamic effects. Therefore, safety switches are being developed to turn off or tune down CAR-T cell killing and allow for more controlled and safer therapies.
Suicide switches are one example of a safety switch where CAR-T cells are further engineered to express “suicide genes” or “elimination genes” which allows selective destruction of CAR-T cells upon administration of an external agent. For example, incorporating herpes simplex virus thymidine kinase (HSV-TK) means that administration of the prodrug ganciclovir results in cell death by incorporation of GCV-triphosphate into replicating DNA. However, the elements involved in this switch are immunogenic and there is emerging evidence that immune responses against HSV-TK limit the persistence of transduced cells Berger et al., (2006) Blood March 15:107(6):2294-302).
WO2017024318 describes compositions and methods for regulating chimeric antigen receptor immune effector cell therapies by attaching a dTAG which binds a heterobifunctional compound which, in turn, leads to ubiquitination.
In order for cellular therapies to be become more widely adopted, there is still a need in the art to develop methods for controlling these therapies to ensure that any adverse events can be prevented.